The author whose themes have been acknowledged by existentialist writers as formative was an obscure nineteenth-century Dane, SØren Kierkegaard (1813-1855). Kierkegaard was a very curious man as well as a prolific writer, but his fame is chiefly as a religious thinker rather than a philosopher in the normal sense. By upbringing and persuasion he was a Protestant Christian, and for a time aspired to be a country parson. Nonetheless he reacted fiercely against many aspects of the Danish Lutheran church of his day. This reaction was volubly expressed in a large number of writings. However, Kierkegaard was also reacting to the philosophy dominant in Northern Europe in the early and middle nineteenth century, namely the philosophy of one of Berlin's most famous professors, G W F Hegel.
Kierkegaard's objections to established Lutheranism and to Hegelian philosophy were at bottom the same. To his mind, both, in different ways, tried to make the demands of Christianity reasonable. In the case of the church, the Gospel was presented, not as a radical challenge to the customary intellectual and social order of the world, but as the sort of thing that reasonable and respectable men and women would naturally agree to. He instances the biblical story of Abraham and Isaac. In that story Abraham, under the belief that God requires it of him, is represented as willing to take an innocent child, his own son, and murder him, though in the end the boy lives. Kierkegaard was struck by the fact that church